In the electroluminescence process of an organic electroluminescence device, luminescence occurs mainly due to the electronic transition of an organic luminescent material from an excited state to a ground state. At room temperature, luminescence generated by the electronic transition from a triplet excited state back to the ground state is extremely weak, most of the energy is lost in the form of heat, and luminescence is mainly generated by the electronic transition from a singlet excitation state to the ground state, and is called electroluminescence. Since the probability of the triplet excited state is three times that of the singlet excitation state, the equivalent of 75% of the energy is not used for luminescence. Making full use of the energy will effectively improve the luminescence efficiency of the organic electroluminescence device.
In order to make full use of the energy of the triplet excited state of a host material of a luminescent layer, various methods have been proposed. For example, by researching and developing an efficient phosphorescence doping dye and doping the phosphorescence doping dye into the host material, the triplet energy of the host material can be effectively transmitted to the phosphorescence doping dye, and then the phosphorescence doping dye generates phosphorescence, thereby allowing the energy of the triplet excited state of the host material of the luminescent layer to be effectively used. The organic electroluminescence device obtained by this method has high efficiency, but material synthesis requires precious metals such as ruthenium and platinum which are expensive. Another method is to utilize the intersystem crossing property of lanthanide compounds, namely intramolecular energy transfer, to transfer the triplet energy of the host material of the luminescent layer to the 4f energy level of lanthanide metal ions, and then luminescence and the like are achieved, but current devices are inefficient.
Thermally activated delayed fluorescence (TADF) is a quite popular scheme utilizing triplet exciton energy at present. For example, Adachi reported in his article a thermally activated delayed fluorescence material which has a small difference (ΔEST) between the triplet energy level (T1) and the singlet energy level (S1), in this way, triplet energy can be transmitted to the singlet energy level, and light is emitted through fluorescent radiation. Patent CN 102709485 A mentions that device efficiency is improved by doping a fluorescent dye in a thermal delay fluorescent host. In order to further improve energy transmission recombination efficiency, in the article “High-efficiency organic light-emitting diodes with fluorescent emitters” by Adachi et al. in Nature communications 2014, a wide-band-gap host doped TADF material is proposed as an auxiliary dye solution. However, in the process of charge recombination, part of the energy is directly compounded on a host, the host transmits the singlet energy to a dye, and the other part is compounded on an auxiliary dye. The structure reported in the article can not fully and effectively utilize the energy directly compounded on the host. At the same time, an ordinary host material is used, a band gap is wide, and required driving voltage is high.
However, the current TADF material has a short lifetime, and one of the reasons is that the lifetime of the triplet state is too long, and exciton quenching can be caused easily due to processes such as TPA. Therefore, the lifetime of a TADF device can be effectively increased by reducing the lifetime of triplet excitons.